The Napoleonic Wars in the US are far from being a mainstream reenactment period, but there are a few groups out there, especially War of 1812 units.
There are also a few Revolutionary war and 7 years war units out there, the latter being quite small in comparison to the former.
And, of course, there are Civil War units all over the place. You should have no problems at all to find one of those, regardless of the state you live in.
Oh, and there are a few WW2 units in the east coast. You can ask Kpetschulat about that.